As a work machine, there has been known a hybrid work machine such as an excavator in which an engine drives a generator motor and a hydraulic pump, whereby an electric motor for an upper rotary body, in which a cab and the like are provided, is driven by electric power generated by the generator motor while a hydraulic actuator for a working equipment and a hydraulic motor for a traveling device are driven by a pressure oil from the hydraulic pump. The electric power generated by the generator motor is charged in a capacitor serving as an electric storage device and is outputted from the capacitor to the electric motor through an inverter. In other words, while a hybrid automobile uses a secondary battery with a high energy density as an electric storage device, a hybrid work machine typically uses a capacitor, which is capable of quickly recovering energy and storing electricity, because an engine speed frequently varies in a short duration of time during an excavation work or the like.
Such a hybrid work machine sometimes uses an auto-idle-stop mechanism to stop the engine when the machine does not operate in order to further reduce fuel consumption. However, in some hybrid work machines, auto idle stop is prohibited when the voltage of an electric storage device falls below a predetermined level (Patent Literature 1).
Similarly, in some hybrid automobiles, auto idle stop is prohibited when a detected temperature of a battery (e.g., a lead-acid battery and a lithium-ion battery) used therein is high (Patent Literature 2). According to Patent Literature 2, when the temperature of the battery is high, a relatively low actual charging capacity is calculated. Therefore, when idle stop is carried out, it is unsure whether or not the battery can supply enough electric power to restart the engine. Accordingly, idle stop is prohibited when the battery temperature is high. Further, when idle stop is prohibited, the engine is continuously driven and the battery is also continuously charged, so that a charging mode is switched to a low-charging mode to suppress a rise in the battery temperature.